2022-02-23 03:03:14

by Paul Moore

[permalink] [raw]
Subject: Re: [PATCH V2] security/selinux: Always allow FIOCLEX and FIONCLEX

On Mon, Feb 21, 2022 at 8:15 AM Richard Haines
<[email protected]> wrote:
>
> These ioctls are equivalent to fcntl(fd, F_SETFD, flags), which SELinux
> always allows too. Furthermore, a failed FIOCLEX could result in a file
> descriptor being leaked to a process that should not have access to it.
>
> As this patch removes access controls, a policy capability needs to be
> enabled in policy to always allow these ioctls.
>
> Based-on-patch-by: Demi Marie Obenour <[email protected]>
> Signed-off-by: Richard Haines <[email protected]>
> ---
> V2 Change: Control via a policy capability. See this thread for discussion:
> https://lore.kernel.org/selinux/[email protected]om/T/#t
>
> With this patch and the polcap enabled, the selinux-testsuite will fail:
> ioctl/test at line 47 - Will need a fix.
>
> security/selinux/hooks.c | 7 +++++++
> security/selinux/include/policycap.h | 1 +
> security/selinux/include/policycap_names.h | 3 ++-
> security/selinux/include/security.h | 7 +++++++
> 4 files changed, 17 insertions(+), 1 deletion(-)

Thanks Richard for putting together the v2 of this patch.

As far as the test is concerned, it seems like the quick-n-dirty fix
is to simply remove the ioctl(FIOCLEX) test in test_noioctl.c; is
everyone okay with that? At least that is what I'm going to do with
my local copy that I use to validate the kernel-secnext builds unless
someone has a better patch :)

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 5b6895e4f..030c41652 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3745,6 +3745,13 @@ static int selinux_file_ioctl(struct file *file, unsigned int cmd,
> CAP_OPT_NONE, true);
> break;
>
> + case FIOCLEX:
> + case FIONCLEX:
> + /* Must always succeed if polcap set, else default: */
> + if (selinux_policycap_ioctl_skip_cloexec())
> + break;
> + fallthrough;
> +

The break/fallthrough looks like it might be a little more fragile
than necessary, how about something like this:

case FIOCLEX:
case FIONCLEX:
if (!selinux_policycap_ioctl_skip_cloexec())
error = ioctl_has_perm(cred, file, FILE__IOCTL, (u16) cmd);
break;

Yes, it does duplicate the default ioctl_has_perm() call, but since we
are effectively deprecating this and locking the FIOCLEX/FIONCLEX
behavior with this policy capability it seems okay to me (and
preferable to relying on the fallthrough).

Thoughts?

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paul-moore.com